Tactile feedback has received growing attention as a means of enhancing or enabling information display in diverse computing applications. As human computer interaction has extended beyond the desktop computing paradigm, and into every other domain of human activity, tactile display has grown in importance. This is attributable, in part, to its ability to overcome the sensory overload and attention demands that arise in complex, multitasking environments.
Two new paradigms that have emerged within this context are those of mobile or wearable information appliances, and of ambient computing. While significant attention has been devoted to the opportunities for tactile display to enhance mobile applications, less has been given to haptic interaction with computationally augmented environments. Nonetheless, the fundamental role that floor surfaces play in our haptic negotiation of everyday environments suggests that they hold significant potential for active tactile information display. The design of haptic information for ground surfaces has a long history, as is evidenced in urban environments. Haptic markers are commonly used to indicate locations or paths of interest to visually impaired people. Similarly, they are employed to emphasize low-lying features, such as subway stairs, that need to be highlighted even to sighted individuals.
While the literature on haptic interaction design for the feet is quite limited, much of what is known about haptic communication via other areas of the body may be readily extended thereto. In addition, in large part through research conducted in the last two decades, more is now known about tactile sensation in the feet.
The tactile sensory physiology and psychophysics of the foot have been the subject of considerable research, if to a lesser extent than in the case of the hand. The foot has, since the mid-20th century, been acknowledged as one of the most sensitive parts of the body to vibrotactile stimulation. Its sensory physiology is similar to that of the hand, including the same types of tactile mechanoreceptors as are present in the hand, namely fast-adapting type I and II receptors (FA I, FA II) and slow-adapting type I and II receptors (SA I, SA II). Their nervous responses largely mirror those of receptors in the hand, although some differences in peripheral vibrotactile information coding have been identified.
Receptor spatial distribution in glabrous skin of the foot sole is relatively widely distributed, with (in contrast to the hand) little preferential accumulation in the toe areas. Receptive fields are larger than in the hand by a factor of about three. Physiological activation thresholds are determined to be higher on average—by a factor of approximately eight, in the case of FA II receptors. It has been suggested that this is due, in part, to biomechanical differences between the skin of the hands and of the feet, and possibly to mechanoreceptor properties. The ball and arch of the foot have been found to be the areas most sensitive to vibrotactile stimulation.
Sensitivity has also been assessed for populations of different ages, with elderly people demonstrating elevated thresholds for vibrotactile stimulation at FA II mediated frequencies (i.e., those most often targeted by vibrotactile displays). Thus, age is a significant factor in haptic interaction design. As in other areas of haptic communication, such differences may be compensated by learning on the part of users, or by plasticity effects, whereby repeated exposure over time has been found to improve vibrotactile discrimination.
Distinctive functional characteristics of the foot relative to the hand include the reduced prehensile dexterity of the former (which is reflected in the kinds of activities in which it is involved), and the fact that static and dynamic forces on the feet during stance are higher and more sustained than those in the hand (i.e., on the order of 100 to 1000 Newtons in the former case). Thus, while the thresholds measured were assessed as subjects were lying down or otherwise off their feet, when individuals are walking, those thresholds may be higher, due to adaptation effects resulting from the large forces involved. As in the case of the hand, most of the receptor types of the foot are simultaneously active during normal motor activities, unlike the more segregated responses that are observed to accompany simpler cutaneous stimulation by static probes, vibrators, or electrodes. Vibrotactile stimulation of the foot can lead to a transformation of physiological messages potentially leading to the overestimation of static forces through co-activation of SA I afferents.
As a result, the application of extrinsic vibrotactile stimulation can result in unintended behavioral modifications affecting posture and gait. Various proprioceptive illusions thus could be induced by vibrotactile stimulation. Humans on foot are implicitly engaged in a sensorimotor task (e.g., quiet stance or normal walking). The cutaneoustactile channels addressed by these types of interfaces are active in the peripheral regulation of balance and locomotion through reflexes coordinating stimuli felt through the feet to muscles in the leg and foot. During locomotion, the coupling of motor reflexes to cutaneous stimulation depends on both stimulus properties and on the instantaneous gait phase at the time of stimulation.
There has been much recent interest in the observation that it is possible to enhance sensation in the feet, and thereby postural and gait control, by providing sub threshold noise to the foot soles. This effect is seen as significant for elderly populations, and for others with peripheral neuropathies.
While there has been little research on the design of haptically actuated floor surfaces, much may be learned from past work in areas such as the passive haptic design of ground surfaces, tactile feedback in foot-based human computer interaction, and locomotion interfaces for virtual environments.
Public transit areas, such as urban sidewalks, pose ample risks to pedestrians under normal sensory conditions. For people with visual impairments, the dangers are amplified, in part because they cannot make use of visual cues or signs that are the most common means of marking hazards (e.g., at intersections). Tactile ground surface indicators consist of regularly textured areas of ground, in the form of patterns of raised domes, bars, or other bumps, arranged on the sidewalk to mark significant paths or points of safety. While international specifications for such markers remain to be established, they must be clearly identifiable, without being obtrusive. When higher than about 5 mm, they have been found to pose risks for stumbling or falling. Alternative means of demarcating floor areas have been proposed to remedy this. For example, the discrimination of floor areas by elasticity, was suggested as a substitute for ground surface indicators.
One area of recent research has concerned the engineering of locomotion interfaces for virtual environments. However, this research has predominantly focused on the challenging problems of kinesthetic (movement) display via high-fidelity force-reflecting haptic interfaces, primarily for omnidirectional virtual walking experiences. Examples include omnidirectional treadmills and robotic foot platforms for simulating walking-in-place. The display of vibrotactile information (i.e., high-frequency force information) underfoot for the purpose of increasing immersion during locomotion in virtual environments has only recently begun to be addressed.
Shoes for conveying vibrotactile information non-interactively (i.e., independent of the actions of their wearers) have, for example, been investigated for information conveyance via non-intrusive or handsfree interfaces. These investigations have found that users were able to identify several families of haptic icons, consisting of moving patterns on the foot sole presented through an array of small vibration motors in the sole of a shoe-like apparatus. Despite the limited number of examples explicitly linked to the feet, there is ample evidence that information can be transmitted via body surfaces, through a range of devices, encodings, and under many different conditions. Beginning in the 1960s, some researchers systematically studied the use of tactile displays for communicating symbolic information via different parts of the body. Later research on sensory substitution aimed at conveying information about shape, spatial configuration, or environmental conditions near a user of a distributed tactile display; such displays were designed for body parts such as the tongue, forehead, thigh, or back.
Basic guidelines for stimulation by vibrotactile feedback are now being developed. Trends in recent research aim at uncovering central capacities for, and limitations on, tactile information, and at establishing a foundation for the design of large sets of structured vibrotactile messages, based on perceptual and usability. Although such guidelines necessarily depend on the display device, application, and user community addressed, basic strategies have been successfully applied to many different interfaces and sensory modalities.
There are many control interfaces for machine operation by foot (car accelerator pedals, dental equipment, sewing machines), and somewhat fewer for human computer interaction (foot controlled computer mice, sensing floors and shoes). Few of these have profited from active haptic feedback. Systems providing haptic warning cues via automobile's accelerator pedal have been researched for many years as means of improving driving safety, and implementations have now reached the market (e.g., Infiniti's Distance Control Assist). Haptic communication during a human control task conducted on a haptically augmented stair climbing machine has also been conducted. Some simple haptic cues supplied to the feet via an exercise machine were found to be effective at aiding participants in maintaining a target exercise level. The cues consisted of regularly spaced tapping sequences encouraging the person exercising to exert more effort when he or she slowed down. Vibrotactile interfaces for furnishing additional feedback during computer music performance, including vibrating floor tiles and in shoe stimulators have also been developed. While the feedback has been found subjectively effective at conveying spatial and temporal information, no systematic evaluation was performed.
There is therefore a need for providing efficient floor-based haptic communication systems, suitable for various types of applications.